Hidalgo County (Texas) Colonias Projects

These projects were conducted under the Economically Distressed Area
Program (EDAP) of the Texas Water Development Board (TWDB). The prime
contractor on these projects was Carter & Burgess, Inc. Venhuizen performed
all work dealing with "innovative/alternative" management options as a
subcontractor to Carter & Burgess. Venhuizen participated in three projects
dealing with Hidalgo County colonias: the Rural Colonias Project, the
scoping phase of the Regional Colonias Project, and the facility planning
phase of the Regional Colonias Project.

The Rural Project entailed detailed system planning for 4 selected colonias,
containing 22, 72, 83 and 84 lots. Venhuizen prepared system plans and
generated cost estimates for the following options for each of these colonias:

Small-scale collective systems in which effluent from septic tanks
on each lot flows through small diameter gravity sewers to multiple
low-pressure-dosed (LPD) dispersal fields scattered around the perimeter
of the colonia.

Small-scale collective systems in which effluent from septic tanks
on each lot flows through small diameter gravity sewers to multiple
sand filter treatment units, the effluent from which is piped to one
or more points where it is disinfected and discharged into an irrigation
drainage canal.

Large-scale collective systems in which effluent from septic tanks
on each lot flows through small diameter gravity sewers to one large
LPD dispersal field.

Large-scale collective systems in which effluent from septic tanks
on each lot flows through small diameter gravity sewers to a single
sand filter treatment plant, the effluent from which is disinfected
and discharged into an irrigation drainage canal.

Large-scale collective systems in which effluent from septic tanks
on each lot flows through small diameter gravity sewers to a single
constructed wetland (gravel marsh) treatment plant, the effluent from
which is disinfected and discharged into an irrigation drainage canal.

These detailed studies provided the basis for a "mainline" strategy for
addressing other similarly situated colonias in the Regional Project.
It was found that the estimated costs of the various strategies did not
vary significantly enough to positively favor any one approach over the
other. Considerations of managing a discharge permit vs. a soil dispersal
system dictated that the LPD system options would be the favored course
of action.

The estimated costs to serve existing houses using these decentralized
concept options in these four colonias ranged from about $3,800 per house
in the colonia with 64 existing units to about $6,000 per house in the
colonia with 16 existing units. Cost per home at buildout ranged from
about $3,300 to about $5,000. Estimated monthly O&M cost for the LPD systems
were less than $10/home/month in all cases. Estimated O&M cost for the
options with sand filter or wetland treatment plants ranged from about
$12/home/month in the larger colonias to over $20/home/month in the smallest
one. Estimated costs of hooking these colonias into a centralized collection
and treatment system was over $11,000 per home in all cases, confirming
that the decentralized concept is far less costly.

In the scoping phase of the Regional Project, approximately 125 colonias
were evaluated for need and eligibility for participation in EDAP. Venhuizen
assisted Carter & Burgess in evaluation of the costs of centralized sewerage
service vs. decentralized management options like those studied in the
Rural Project. Approximately 30 colonias were selected for inclusion in
the facility planning phase of the Regional Project. Of those, 16 were
determined to be definitely better suited to a decentralized management
plan. Venhuizen was totally responsible for generating the facility plans
for that group. These 16 colonias range in size from 18 to 114 lots. The
total number of lots in this group is 737, yielding an average of 46 lots
per colonia. Another 10 colonias, ranging in size from 17 to 96 lots (containing
478 total lots for an average of 48 lots per colonia), were evaluated
for both conventional sewerage service and a decentralized management
plan. Venhuizen generated the system plans and cost estimates for the
decentralized option for those 10 colonias. The remainder of these colonias
were considered to be close enough to an existing sewer line that centralized
sewerage would be the least cost solution.

The results of Venhuizen's analysis indicated that these colonias could
be serviced using the LPD system concept with a net present worth of construction
cost in the range of $3,106-$4,594 per lot, including all planning, permitting,
engineering, land acquisition and legal costs. The high cost was an "outlyer"-the
next highest cost per lot was $4,118. The weighted average cost per lot
was $3,527. The figure below shows the schematic system layout used to
generate these cost estimates for one of these colonias. Layouts for all
the colonias are similar.

O&M costs were also evaluated for the 16 lots in the first group and,
based on the assumptions made, yielded costs in the range of $5.37 to
$8.69 per lot per month. The weighted average among this group was $6.17
per lot per month. Cost factors included were inspections, electricity,
an allowance for general maintenance, pump replacement (7-year intervals
assumed), and pumping of septic tanks and dosing tanks.

As a comparison of these costs to the costs of hooking into a conventional,
centralized collection and treatment system, the total cost for connecting
7 of these colonias was estimated at $2.08 million. The total cost of
the decentralized systems for these seven colonias was estimated at $1.23
million, 59% of the cost of the centralized strategy. Since these colonias
were situated much more favorably for connecting to the treatment plant
than the 16 colonias in the first group, it is to be expected that there
would be a much greater cost differential for that group.

These facility plans were completed in 1998 and are currently under review
by the TWDB. The local sponsor of most of the decentralized projects,
North Alamo Water Supply Corporation, has favorably received the decentralized
management concept and has exhibited a readiness to build and manage these
systems once these plans are accepted by TWDB. Toward that end, Venhuizen
has worked with North Alamo staff to generate a management plan for system
surveillance and maintenance.